From Palladium to Brønsted Acid Catalysis: Highly Enantioselective Regiodivergent Addition of Alkoxyallenes to Pyrazolones

Catalytic Enantioselective Propargylation of Pyrazolones by Amide-Based Phase-Transfer Catalysts

In this paper, we developed a highly enantioselective alkylation of 4-substituted pyrazolones catalyzed by phase-transfer catalysis. Cheap halohydrocarbons were employed as electrophilic alkylationg agents, and propargyl, allyl, and benzyl products with all-carbon quaternary stereocenters were afforded with excellent enantioselectivities and good yields. We found that the unique structures of the catalyst (hydrogen bond donors of the C-9 hydroxyl group and amide group, the triphenyl at the NH-position) were important for good enantioselectivity. Furthermore, chiral propargyl products could be easily connected to azide molecules by click cycloaddition, which offers unique opportunities to obtain structurally diverse chiral pyrazolones.

Asymmetric bifunctionalization of allenes with aryl iodides and amino acids enabled by chiral aldehyde/palladium combined catalysis

Even though catalytic asymmetric bifunctionalization of allenes has been extensively studied, almost all of the reported examples have been achieved in a two-component manner. In this study, we report a highly efficient asymmetric bifunctionalization of allenes with iodohydrocarbons and NH2-unprotected amino acid esters. The adopted chiral aldehyde/palladium combined catalytic system precisely governs the chemoselectivity, regioselectivity, and stereoselectivity of this three-component reaction. A wide range of substituted aryl iodides, allenes and amino acid esters can well participate in this reaction and deliver structurally diverse α,α-disubstituted α-amino acid esters with excellent experimental outcomes. One of the resulting products is utilized for the total synthesis of the molecule (S,R)-VPC01091.

Asymmetric Synthesis of Trisubstituted Vicinal Diols through Copper(I)-Catalyzed Diastereoselective and Enantioselective Allylation of Ketones with Siloxypropadienes

Chiral trisubstituted vicinal diols are a type of important organic compounds, serving as both common structure units in bioactive natural products and chiral auxiliaries in asymmetric synthesis. Herein, by using siloxypropadienes as the precursors of allyl copper(I) species, a copper(I)-catalyzed diastereoselective and enantioselective reductive allylation of ketones was achieved, providing both syn-diols and anti-diols in good to excellent enantioselectivity. DFT calculations show that cis-γ-siloxy-allyl copper species are generated favorably with either 1-TBSO-propadiene or 1-TIPSO-propadiene. Moreover, the steric difference of TBS group and TIPS group distinguishes the face selectivity of acetophenone, leading to syn-selectivity for 1-TBSO-propadiene and anti-selectivity for 1-TIPSO-propadiene. Easy transformations of the products were performed, demonstrating the synthetic utility of the present method. Moreover, one chiral diol prepared in the above transformations was used as a suitable organocatalyst for the catalytic asymmetric reductive self-coupling of aldimines generated in situ with B2(neo)2.

A copper-catalyzed asymmetric Friedel-Crafts hydroxyalkylation of pyrazole-4,5-diones with 5-aminoisoxazoles

An asymmetric Friedel-Crafts hydroxyalkylation reaction of 5-aminoisoxazoles with pyrazole-4,5-diones was developed under the catalysis of 5% chiral copper complexes. This reaction exhibits functional group tolerance and excellent enantioselectivity. Moreover, the reaction can be scaled up and its mechanism was studied.

Pd-Catalysed asymmetric allylic alkylation of heterocycles: a user's guide

This review provides an in-depth analysis of recent advances and strategies employed in the Pd-catalysed asymmetric allylic alkylation (Pd-AAA) of nucleophilic prochiral heterocycles. The review is divided into sections each focused on a specific family of heterocycle, where optimisation data and reaction scope have been carefully analysed in order to bring forward specific reactivity and selectivity trends. The review eventually opens on how computer-based technologies could be used to predict an ideally matched catalytic system for any given substrate. This user-guide targets chemists from all horizons interested in running a Pd-AAA reaction for the preparation of highly enantioenriched heterocyclic compounds.

Synergistic Catalysis Involving Palladium for Stereodivergent Csp3-Csp3 Coupling Reactions

ConspectusTransition-metal-catalyzed coupling reactions of dienes (such as 1,3-dienes, alkoxyallenes, and aminoallenes) with carbon nucleophiles have proven to be a highly effective method for creating Csp3-Csp3 bonds. These reactions have perfect atom economy and typically occur under mild reaction conditions. By using chiral metal complexes as catalysts, it is possible to create enantioenriched molecules bearing allylic stereocenters with high enantioselectivities. However, challenges arise when Csp3-Csp3 bonds bearing two vicinal stereocenters are constructed through this type of coupling reaction. Due to the inherent diastereoselectivities, only the kinetically favored diastereoisomers (either the syn- or anti-product) are usually obtained through the transition-metal catalyst system. Achieving complementary stereoisomers with high selectivity, which require complete control of both absolute and relative configurations of multiple chiral centers in a single chemical transformation, is usually impossible.Over the past decade, significant advancements have been made in stereodivergent synthesis. Notably, iridium-related synergistic catalysis has been rapidly developed for stereodivergent allylic alkylation reactions. However, these systems were limited to using allylic alcohol derivatives as electrophilic partners. Finding ways to extend the use of synergistic catalysis to other types of stereodivergent reactions is a crucial issue that needs to be addressed.In 2019, we reported the first palladium-mediated synergistic system for the stereodivergent Csp3-Csp3 coupling between 1,3-dienes and aldimine esters. Lately, this strategy has proven successful in accessing stereodivergent coupling with diverse substrate patterns. In this Account, we will summarize our laboratory's efforts in developing a range of palladium-involved synergistic catalysis systems for the stereodivergent Csp3-Csp3 coupling reactions of dienes. We discovered several synergistic catalysis systems, including Pd/Cu(Ag), Pd/amine, Pd/Lewis base, and Pd/PTC. Additionally, we developed diverse dienes, such as 1,3-dienes, alkoxyallenes, and aminoallenes, to serve as suitable coupling partners for stereodivergent coupling. These processes provide an efficient method for constructing a range of chiral scaffolds bearing vicinal stereocenters. Density functional theory (DFT) calculations have been performed to elucidate the reaction mechanism and to rationalize the origins of the stereochemistry for some of the synergistic catalyst systems. Finally, the synthetic application of these methods has been demonstrated in the concise total synthesis of a number of natural products and bioactive molecules. It is anticipated that an increasing number of chemists will join in the research on stereodivergent Csp3-Csp3 coupling reactions and contribute to more elegant examples in this area. We believe future development will further push the boundary of asymmetric catalysis and find more innovative applications soon for synthesizing complex chiral molecules.

Access to Chiral O,O-Acetals Enabled by Palladium-Catalyzed Asymmetric Addition of Oximes to Alkoxyallenes

Enantiomerically pure acyclic O,O-acetal compounds (up to 97 % ee) have been accessed through chemo-, regio- and enantioselective palladium-catalyzed addition of oximes to alkoxyallenes. DFT calculations support that a protonative hydropalladation pathway is favourable, in which the hydrogen bonding interaction between the amide group of the diphosphine ligand and the alkoxyallene is critical for the highly stereoselective formation of the dioxygenated stereogenic center.

Metal-Catalyzed Enantioconvergent Transformations

Enantioconvergent catalysis has expanded asymmetric synthesis to new methodologies able to convert racemic compounds into a single enantiomer. This review covers recent advances in transition-metal-catalyzed transformations, such as radical-based cross-coupling of racemic alkyl electrophiles with nucleophiles or racemic alkylmetals with electrophiles and reductive cross-coupling of two electrophiles mainly under Ni/bis(oxazoline) catalysis. C-H functionalization of racemic electrophiles or nucleophiles can be performed in an enantioconvergent manner. Hydroalkylation of alkenes, allenes, and acetylenes is an alternative to cross-coupling reactions. Hydrogen autotransfer has been applied to amination of racemic alcohols and C-C bond forming reactions (Guerbet reaction). Other metal-catalyzed reactions involve addition of racemic allylic systems to carbonyl compounds, propargylation of alcohols and phenols, amination of racemic 3-bromooxindoles, allenylation of carbonyl compounds with racemic allenolates or propargyl bromides, and hydroxylation of racemic 1,3-dicarbonyl compounds.

Diastereodivergent Aldol-Type Coupling of Alkoxyallenes with Pentafluorophenyl Esters Enabled by Synergistic Palladium/Chiral Lewis Base Catalysis

As a fundamental and synthetically useful C-C bond formation reaction, the aldol reaction is one of the most versatile transformations in organic synthesis. However, despite extensive research on asymmetric versions of the reaction, a unified method for stereoselective access to the complementary syn and anti diastereomeric products remains to be developed. In this study, we developed a synergistic palladium/chiral Lewis base system that overcomes the inherent diastereoselectivity bias of aldol reactions and, as a result, allowed us to achieve the first diastereodivergent coupling reactions of alkoxyallenes with pentafluorophenol esters. Computational studies suggest a mechanism involving an intermolecular protonative hydropalladation pathway rather than a palladium-hydride migratory insertion pathway. The origin of the stereochemistry for this synergistic catalysis system is rationalized by DFT calculations.

A Pd-H/Isothiourea Cooperative Catalysis Approach to anti-Aldol Motifs: Enantioselective α-Alkylation of Esters with Oxyallenes

The biological and therapeutic significance of natural products is a powerful impetus for the development of efficient methods to facilitate their construction. Accordingly, and reflecting the prevalence of β-oxy-carbonyl motifs, a sophisticated arsenal of aldol-based strategies has evolved that is contingent on the generation of single enolate isomers. Since this has the potential to compromise efficiency in reagent-based paradigms, direct catalysis-based solutions would be enabling. To complement the array of substrate-based strategies, and regulate enolate geometry at the catalyst level, a direct catalytic alkylation of esters with oxyallenes has been developed. Synergizing metal hydride reactivity with Lewis base catalysis has resulted in a broad reaction scope with useful levels of stereocontrol (up to >99 % ee). Facile derivatization of these ambiphilic linchpins is demonstrated, providing access to high-value vicinal stereocenter-containing motifs, including 1,2-amino alcohols.

Brønsted Acid Catalyzed Direct Annulation of Alkoxyallenes and Naphthols to Chroman Ketals

A straightforward Brønsted acid-catalyzed and scalable annulation of alkoxyallenes with simple naphthols was developed, affording chroman ketals in 49–84% yields. The versatile chroman ketals can be easily converted into coumarins by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-mediated oxidation, and a series of 2-substituted chromans via nucleophilic substitutions.

Chiral phosphoric acid-catalyzed regio- and enantioselective reactions of functionalized propargylic alcohols

Chiral phosphoric acid has been utilized for covalent activation of propargylic alcohols to act as pre-catalyst. With this activation mode, a range of highly regio- and enantioenriched heterocyclic products could be generated efficiently from racemic propargylic alcohols.

C-4 benzofuranylation of pyrazolones by a metal-free catalyzed indirect heteroarylation strategy

A metal-free catalyzed indirect heteroarylation of pyrazolones with 2-(3-hydroxy-3,3-diarylprop-1-yn-1-yl) phenols has been developed, and a series of novel 4-benzofuranyl substituted pyrazolone derivatives were obtained in moderate to excellent yields (up to 90%). The process has the salient features of metal-free catalysis, operational simplicity, good substrate compatibility and mild reaction conditions. In particular, the integration of the pyrazolone skeleton and benzofuran scaffold into a single molecule is expected to be of potential interest for medicinal research. In addition, the yield and efficiency are basically maintained in the gram-scale experiment, which makes the practical application of the process more prominent.

Asymmetric construction of acyclic quaternary stereocenters via direct enantioselective additions of α-alkynyl ketones to allenamides

Acyclic quaternary stereocenters are widely present in a series of biologically active natural products and pharmaceuticals. However, development of highly efficient asymmetric catalytic methods for the construction of these privileged motifs represents a longstanding challenge in organic synthesis. Herein, an efficient chiral phosphoric acid catalyzed direct asymmetric addition of α-alkynyl acyclic ketones with allenamides has been developed, furnishing the acyclic all-carbon quaternary stereocenters with excellent regioselectivities and high enantioselectivities. Extensive and detailed experimental mechanistic studies were performed to investigate the mechanism of this reaction. Despite a novel covalent allyl phosphate intermediate was found in these reactions, further studies indicated that a SN2-type mechanism with the ketone nucleophiles is not very possible. Instead, a more plausible mechanism involving the elimination of the allyl phosphate to give the α,β-unsaturated iminium intermediate, which underwent the asymmetric conjugate addition with the enol form of ketone nucleophiles under chiral anion catalysis, was proposed. In virtue of the fruitful functional groups bearing in the chiral products, the diverse derivatizations of the chiral products provided access to a wide array of chiral scaffolds with quaternary stereocenters.

Palladium-Catalyzed Asymmetric Decarboxylative Addition of β-Keto Acids to Heteroatom-Substituted Allenes

The Pd-catalyzed asymmetric addition reaction of β-keto acids to heteroatom-substituted allene is reported. This reaction generates β-substituted ketones in an asymmetric manner through a branch-selective decarboxylative allylation pathway. The reaction accommodates various alkoxyallenes as well as amidoallenes.

Enantioselective Palladium-Catalyzed Hydrophosphinylation of Allenes with Phosphine Oxides: Access to Chiral Allylic Phosphine Oxides

A Pd-catalyzed hydrophosphinylation of alkyl and aryl-oxyallenes with phosphine oxides has been developed for the efficient and rapid construction of a family of chiral allylic phosphine oxides with a diverse range of functional groups. This methodology was further applied in the facile construction of chiral 2H-chromene and later stage functionalization of cholesterol.

Direct access to spirocycles by Pd/WingPhos-catalyzed enantioselective cycloaddition of 1,3-enynes

Spirocycles play an important role in drug discovery and development. The direct, catalytic, and enantioselective synthesis of spirocycles from readily available starting materials and in an atom economic manner remains a highly sought-after task in organic synthesis. Herein, an enantioselective Pd-hydride-catalyzed cycloaddition method for the synthesis of spirocyclic compounds directly from two classes of commonly available starting materials, 1,3-enynes and cyclic carbon−hydrogen (C−H) bonds, is reported. The reactions employ a chiral Pd/WingPhos catalyst to both suppress the formation of bis-allenyl by-products and control the stereoselectivity. 1,3-Enynes are used as dielectrophilic four-carbon units in the cycloaddition reactions, which also enables an enyne substrate-directed enantioselectivity switch with good levels of stereocontrol. The present spirocycle synthesis tolerates a broad range of functional groups of 1,3-enyne substrates, including alcohols, esters, nitriles, halides, and olefins. A variety of diverse cyclic nucleophiles, including pharmaceutically important heterocycles and carbocycles, can be flexibly incorporated with spiro scaffolds.

Spirocycles are traditionally difficult structures to synthesize due to the congested nature of the central atom. Here the authors show a method to synthesize quaternary carbon spirocycles in one step from 1,3-enynes and pyrazolidine-type heterocycles, both relatively unactivated structures, proceeding via palladium catalysis.

Palladium-Catalyzed Asymmetric Trifluoromethylated Allylic Alkylation of Pyrazolones Enabled by α-(Trifluoromethyl)alkenyl Acetates

The first asymmetric trifluoromethylated allylic alkylation of pyrazolones using α-(trifluoromethyl)alkenyl acetates as a novel trifluoromethylated allylation reagent is described, affording various functionalized chiral pyrazolones containing a trifluoromethylated allyl substituent in high yields with excellent regio-/enantio-/diastereoselectivities. Mechanistically, the double-bond migration of α-(trifluoromethyl)alkenyl acetates in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene is initial and interesting step. More importantly, this study is of significance in providing a novel and widely applicable trifluoromethyl-containing allylation reagent.

Synergistic Pd/Amine-Catalyzed Stereodivergent Hydroalkylation of 1,3-Dienes with Aldehydes: Reaction Development, Mechanism, and Stereochemical Origins

Metal-hydride-catalyzed hydroalkylation of 1,3-dienes with enolizable carbonyl compounds is an atom- and step-economical method for preparing chiral molecules with allylic stereocenters. Although high diastereo- and enantioselectivities have been achieved for many coupling partners, aldehydes have not yet been used for this purpose because they are less stable than other carbonyl compounds under basic conditions and they have the potential to rapidly epimerize at the α-position. Moreover, stereodivergent hydroalkylation reactions of 1,3-dienes to access complementary diastereomers with vicinal stereocenters is challenging. Herein, we describe a synergistic palladium/amine catalyst system that allowed us to achieve the first stereodivergent hydroalkylation reactions of 1,3-dienes with aldehydes. By choosing an appropriate combination of chiral palladium and amine catalysts, we could obtain either syn or anti coupling products, and this method therefore provides highly diastereo- and enantioselective access to complementary diastereomers of chiral aldehydes with α,β-vicinal stereocenters. Density functional theory calculations revealed a mechanism involving PdH formation and migratory insertion into the alkene, followed by C-C bond formation. The origin of the stereoselectivities was investigated by means of distortion/interaction analysis.

Enantioselective Synthesis of Quaternary Carbon Stereocenters by Asymmetric Allylic Alkylation: A Review

Quaternary stereocenters are of great importance to the three-dimensionality and enhanced properties of new molecules, but the synthetic challenges in creating quaternary stereocenters greatly hinder their wide use in drug discovery, organic material design, and natural product synthesis. The asymmetric allylic alkylation (AAA) of allylic substrates has proven to be a powerful methodology for enantioselective formation of structure skeletons bearing single or more quaternary carbon centers in modern asymmetric organocatalysis. AAA has certain advantages in constructing the tetrasubstituted stereocenters, including but not limited to mild reactive conditions, effective reaction rates, new functional group introduction, and carbon chains length extension. This review outlines the key considerations in the application of AAA reactions and summarizes the recent progress of AAA reactions in the enantioselective synthesis of products containing quaternary stereocenters. Meanwhile, a detailed discussion of the AAA reactions such as ligands, scope of substrates, transformations and the general reaction mechanisms is also provided. We hope this review could stimulate further advances in much broader areas, including organic synthesis, asymmetric catalysis, C-H activation, and symmetrical pharmaceutical chemistry.

Stretchable chiral pockets for palladium-catalyzed highly chemo- and enantioselective allenylation

Pyrazolones are a vital class of heterocycles possessing various biological properties and much attention is paid to the diversified synthesis of enantiopure pyrazolone derivatives. We describe here the development of diphenylphosphinoalkanoic acid based chiral bisphosphine ligands, which are successfully applied to the palladium-catalyzed asymmetric allenylation of racemic pyrazol-5-ones. The reaction affords C-allenylation products, optically active pyrazol-5-ones bearing an allene unit, in high chemo- and enantioselectivity, with DACH-ZYC-Phos-C1 as the best ligand. The synthetic potential of the C-allenylation products is demonstrated. Furthermore, the enantioselectivity observed with DACH-ZYC-Phos-C1 is rationalized by density functional theory studies.

Chiral pyrazolone derivatives show promising biological activity in commercial drugs. Here, the authors report an enantioselective allenylation of pyrazolones by fine tuning of Trost’s ligands, which leads to a chiral pocket featuring high efficiency and asymmetric induction in the catalytic process.

Electrochemical synthesis of versatile ammonium oxides under metal catalyst-, exogenous-oxidant-, and exogenous-electrolyte-free conditions

An electrochemical oxidative cross-coupling reaction between 2.5-substituted-pyrazolin-5-ones and ammonium thiocyanate has been developed, which resulted in a series of unprecedented cross-coupling products under metal catalyst-, exogenous-oxidant-, and exogenous-electrolyte-free conditions. It is worth noting that since the resulting cross-coupling products are nearly insoluble in MeCN, the pure product could be afforded without silica gel column purification. In addition, the prepared ammonium oxides are versatile building blocks for synthesizing functionalized pyrazole derivatives.

Catalytic asymmetric construction of C-4 alkenyl substituted pyrazolone derivatives bearing multiple stereoelements

An organocatalytic asymmetric process was reported for the sterically precise construction of C-4 alkenyl substituted pyrazolone derivatives bearing multiple stereoelements.

Enantioselective Construction of Quaternary Stereogenic Centers by the Addition of an Acyl Anion Equivalent to 1,3-Dienes

We report the enantioselective formation of quaternary stereogenic centers by the intermolecular addition of malononitrile, an acyl anion equivalent, and related pronucleophiles to several 1,3-disubstituted acyclic 1,3-dienes in the presence of a Pd-PHOX catalyst. Products are obtained in up to 88% yield and 99:1 er and in most cases are formed as a single regioisomer. The products' malononitrile unit undergoes oxidative functionalization to afford β,γ-unsaturated carbonyls bearing internal olefins and α-quaternary stereogenic centers.

Enantio- and Diastereoselective Synthesis of Homoallylic α-Trifluoromethyl Amines by Catalytic Hydroalkylation of Dienes

We describe a strategy for the enantio- and diastereoselective synthesis of homoallylic α-trifluoromethyl amines by the catalytic hydroalkylation of terminal dienes. Trifluoromethyl-substituted isatin-derived azadienolate nucleophiles undergo γ-selective alkylation with a Pd-DTBM-SEGPHOS catalyst, which additionally promotes regioselective addition to the diene and delivers products in up to 86% yield, 10:1 dr, and 97.5:2.5 er.

Palladium-catalyzed diastereo- and enantioselective allylic alkylation of oxazolones with 1,3-dienes under base-free conditions

Herein, we report a highly diastereo- and enantioselective allylic alkylation of oxazolones with 1,3-dienes by palladium-hydride catalyst under base-free conditions.

Rhodium-catalyzed regio- and enantioselective allylic alkylation of pyrazol-5-ones with alkynes

An atom-economical, regio- and enantioselective allylic alkylation of pyrazol-5-ones with alkynes was developed under rhodium catalysis.

Asymmetric synthesis of allylic compounds via hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes

This review article provides an overview of progress in asymmetric synthesis of allylic compounds via hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes.